Presses, shears and other large equipment where work pieces are fed in and out of the closing parts are commonly powered by compressed air, water, steam or hydraulics. The machine may be ready to close but will not do so until the guard moves to the protecting position and the operator actuates START. There is commonly an electrical safety circuit incorporating an emergency STOP which permits operation of a valve which in turn admits fluid to begin closing the machine parts on the work piece. Occasionally the fluid valve fails to close fully and this condition may go undetected until an accident draws attention to it.
If a pair of valves are used to reduce the risk of failure, it is found in practice that if one of the valves malfunctions and sticks in the OPEN position, the remaining valve may continue independently leaving the operator unaware that the system needs investigation. When the second valve fails and an accident follows, it is at that stage that the failure of both valves is discovered. In production work where the machine fails to respond to a START signal and then to a RESET signal, it is useful if some indication be given to the maintenance engineer which component to check. If the system does not monitor the condition of the valves a comprehensive diagnostic procedure is required and this adds to the downtime.
Safety circuits in the art attempt to deal with the danger, but effectiveness depends firstly on adapting a device to the existing circuitry, and secondly upon meeting prevailing standards and obtaining accreditation from safety authorities where applicable. Industry regulations commonly stipulate the use of well tried components and any safety equipment gains approval only if it is built up from such components. Thus the relays, switches, power supplies and the like must all be approved and any advance in safety can only proceed through the interaction of such predictable components.